New wireless electronics could heal wounds and then dissolve

Nestled inside a wound, a remote-controlled device perks up and
begins releasing bacteria-killing heat, a form of thermal therapy
that can fell even the most drug-resistant microbes. After it does
its job, the electronic heater dissolves, and its
biocompatible ingredients become part of the person it has helped
to heal.

Though not quite a reality yet, this scenario isn't too far off.
In addition to dissolvable electronics, US mechanical engineers
have now built a biodegradable remote-controlled, power-harvesting
circuit, described 17 May in Advanced Materials, and
are already testing absorbable thermal electronics in rodents.

This biocompatible remote-controlled circuit is an important
step toward building dissolvable electronics that could function as
"electroceuticals," devices that perform therapeutic roles and then
disappear. Such roles could include stimulating nerve and bone
growth, helping heal wounds, delivering drugs, or acting as
antibiotics.

"In each case, the device needs to function only for a timeframe
set by a healing process. As such, the ideal scenario is for the
device to simply disappear afterward," said John Rogers, a
mechanical engineer at the University of Illinois. Last year,
Rogers described the development of a water-soluble, silicon-based
circuit that completely dissolves in water; earlier this year, his
team produced
tiny LEDs that can be injected into the brain.

The remote-controlled circuits are fashioned on super-thin silk
and are responsive to radio frequencies. The team builds the
capacitors, inductors, and resistors using water-soluble and
biocompatible materials: silicon nanomembranes, which work as
semiconductors; magnesium, which already plays an important role in
biological systems; silicon dioxide or magnesium oxide as
insulators; and silk, for the substrate upon which the circuits are
crafted.

The system's antenna -- a crucial component for receiving the
radio signals used to power the device -- is made by layering
magnesium onto silk. An ultra-thin version, with a 500-nm thick
magnesium antenna, completely dissolves after two hours in
deionised water at room temperature. A version that's six times
thicker can take a few days to dissolve.

To demonstrate the functionality of the device, Rogers and his
colleagues built a power-harvesting circuit that attached the
magnesium-on-silk antenna to an LED. When they switched on a radio
transmitter placed as far as 1.8 metres away, the device converted
about 15 percent of the radio waves it received into electrical
energy, and the light blinked on. Then, when they placed the
circuit in deionised water, it dissolved.

The work is well done and is an important step toward realising
biodegradable electronic systems, said Christopher
Bettinger, a polymers engineer at Carnegie Mellon University.
Bettinger notes that using radio as a power source means that
devices meant to be implanted deeper will need bigger antennas. "I
think the power source is going to be the real issue with
biodegradable electronics," he said. "There are many great
applications, but defining the specific disease or indication where
biodegradable electronics has specific advantage will also be a key
part of the broader strategy."

Now, Rogers and his colleagues are testing a device capable of
delivering thermal therapy in rodents. They've implanted the
ephemeral electronics in approximately 100 mice so far, just
beneath the skin. Using an infrared camera, the scientists can
monitor whether the devices are working. When they are, they raise
the temperature at the implant site by just a few degrees. And so
far, Rogers says, there have been "no signs of inflammation,
fibrosis, or any other kind of adverse reaction," throughout
the course of surgical implantation and reabsorption.